Colloid-facilitated Radionuclide Transport in High-level Nuclear Waste Repository: A Review of Experimental Studies

This review highlights colloid-facilitated transport of radionuclides as a critical factor in evaluating the long-term safety of deep geological repositories (DGRs) for high-level radioactive waste (HLW). Colloids, originating from engineered barriers such as bentonite and from natural geological formations, can remain mobile under certain hydrogeochemical conditions and act as carriers for strongly sorbing radionuclides. This review synthesizes understanding of colloid generation, stability, and transport mechanisms, drawing on laboratory- and field-scale studies. Laboratory investigations have elucidated the roles of colloid size, water chemistry, and organic matter in governing stability and radionuclide interactions, while field observations provide direct evidence of colloid-mediated transport under repository conditions. Comparisons between controlled experiments and in-situ studies underscore the importance of fracture heterogeneity and transient geochemical conditions in shaping transport behavior. Finally, key challenges are discussed, including extrapolating laboratory findings to repository timescales and integrating colloid processes into performance assessment models.